Effect of Increase in Cost of Insurance and Claims (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 27, 2020	Jun. 29, 2019	Jun. 27, 2020	Jun. 29, 2019
Change in Accounting Estimate [Line Items]
Operating income	$ 32,181	$ 80,851	$ 86,186	$ 161,747
Net income attributable to Landstar System, Inc. and subsidiary	$ 24,254	$ 61,100	$ 65,149	$ 124,417
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.63	$ 1.53	$ 1.68	$ 3.11
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.63	$ 1.53	$ 1.68	$ 3.11
Development of Prior Year Self Insured Claims Estimates
Change in Accounting Estimate [Line Items]
Operating income	$ 2,816	$ 1,493	$ 5,039	$ 2,900
Net income attributable to Landstar System, Inc. and subsidiary	$ 2,135	$ 1,131	$ 3,820	$ 2,198
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.06	$ 0.03	$ 0.10	$ 0.05
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.06	$ 0.03	$ 0.10	$ 0.05